1. Field of the Invention
The present invention relates to a multi-beam write image forming apparatus for optically writing using a plurality of light beams.
2. Related Background Art
In a conventional multi-beam write scheme, a plurality of semiconductor lasers are used, and laser beams emitted by these semiconductor lasers are simultaneously scanned in the main scanning direction on a photosensitive body to simultaneously write a plurality of lines. It is important in such write scheme to align the start points of the individual beams in the main scanning direction so as to obtain a high-precision recorded image. In a conventional system, for example, two laser beams reflected by a rotary polygonal mirror 100 are split by a polarization beam splitter 101, and the split laser beams are respectively detected by photodetection elements 102 and 103, respectively. The signals output from these two photodetection elements 102 and 103 are used as reference timings of the individual laser beams, and a write starts on a photosensitive body a predetermined period of time after the photodetection elements 102 and 103 detect the laser beams.
However, in such detection method, a plurality of optical elements such as a polarization beam splitter, photodetection elements, and the like are required, and the apparatus arrangement is complicated. Also, depending on the setup angle of the polarization beam splitter and mounting precision of the photodetection elements, the time from the detection of the two laser beams to the write start positions changes. For this reason, upon mounting these polarization beam splitter and photodetection elements, strict mounting precision is required, resulting in time-consuming mounting.
Also, since the amount of light that becomes incident on each photodetection element changes due to variations in reflectance and contaminations of the respective reflection surfaces of the rotary polygonal mirror, the time required until the threshold value of each photodetection element is exceeded changes, and the generation timing of the output signal of each photodetection element also changes. However, when the timing of the output signal of the photodetection element has changed, since the reference timings of two lines are determined by the output signals from the respective photodetection elements, the write start positions of the two laser beams in the main scanning direction deviate, and the start points in the main scanning direction cannot be accurately aligned.
The present invention has been made in consideration of the conventional problems, and has as its object to provide an image forming apparatus which can obtain a high-precision recorded image free from any deviation of a plurality of write lines with a simple arrangement.
The object of the present invention is achieved by an image forming apparatus, which comprises a plurality of laser sources and writes a plurality of lines on an image carrier by simultaneously scanning laser beams emitted by the respective laser sources in the main scanning direction, characterized by comprising two light-receiving elements juxtaposed in the main scanning direction to detect the laser beams emitted by the plurality of laser sources prior to scanning in the main scanning direction, means for turning on spots formed by the laser beams from the plurality of laser sources in the order from a leading spot, and means for comparing output signals from the two light-receiving elements, and outputting a reference timing signal indicating that each spot has reached a reference position when the spot is located at substantially the middle position between the two light-receiving elements, and in that write operations of the plurality of laser sources in the main scanning direction are synchronized on the basis of the output reference timing signal of the spots.
An image forming apparatus according to the present invention is characterized by comprising:
a plurality of laser sources;
scanning means for scanning laser beams emitted by the plurality of laser sources onto a recording medium;
two light-receiving elements juxtaposed in a main scanning direction to detect the laser beams scanned by the scanning means;
means for comparing output signals from the two light-receiving elements, and outputting a reference timing signal indicating that each of the scanned laser beams has reached a substantially middle position between the two light-receiving elements; and
control means for synchronizing write operations of the plurality of laser sources in the main scanning direction on the basis of the reference timing signal.
The image forming apparatus according to the present invention is characterized in that the two light-receiving elements detect the laser beams scanned by the scanning means prior to scanning on the recording medium in the main scanning direction.
The image forming apparatus according to the present invention is characterized in that when the laser beams scanned by the scanning means pass surfaces of the two light receiving elements, the plurality of laser sources are turned on in an order from a leading laser beam.
The image forming apparatus according to the present invention is characterized in that a downstream one of the two light-receiving elements in the main scanning direction has a width smaller than an upstream light-receiving element.
The image forming apparatus according to the present invention is characterized in that the two light-receiving elements comprise a 2-split photodetection element split in the main scanning direction.
The image forming apparatus according to the present invention is characterized in that when each of the laser beams scanned by the scanning means has exceeded the substantially middle position between the two light-receiving elements, the scanned laser beam is turned off.
The image forming apparatus according to the present invention is characterized in that the two light-receiving elements include upstream and downstream light-receiving elements, and
when each of the laser beams scanned by the scanning means falls substantially outside the upstream light-receiving element, the scanned laser beam is turned off.